In connection with insecticide control, it is well known that insecticides can be integrated in polymer material in order to prevent insects to work their way through the material. However, when it comes to the control of release rates of insecticides in a polymer matrix, this is regarded as a difficult issue, because a release that is too rapid exhausts the matrix quickly with regard to insecticidal effect, and a too slow release does not have the desired efficiency or suitable regeneration properties in case the insecticide falls off or is removed, for instance, by washing.
For crop sheets, as disclosed in U.S. Pat. No. 4,888,174, it is known to mix LDPE with other polymers in order to reduce the migration speed. This is in line with the disclosure of International Patent Application WO 02/34487, stating that “the selection of the polymers depends on the desired release rate”, where the disclosure proposes a pesticide-retaining layer of LDPE (low density polyethylene) bonded to a layer of HDPE (high density polyethylene), where the HDPE layer prevents a too fast release of the insecticide.
For cables, it is known from U.S. Pat. No. 4,680,328 to extrude insecticidal polymer blends with LDPE, HDPE and LLDPE (linear low density polyethylene) and a preferred insecticide halopyridyl phosphate.
Apart from the difficulty of achieving a suitable migration speed of insecticide in polymer matrices, the combination of an insecticide, for example DM (Deltamethrin), and a synergist, for example PBO (piperonyl butoxide), incorporated in a polymer matrix implies additional challenges. Despite the fact that incorporation of a pyrethroid and PBO has been reported in connection with pet collars, for example in International patent application WO 00/40084 or WO 06/127407, the migration speed for long lasting insecticidal nets (LLIN) or fabrics is still an unresolved problem in practice. This is due to the fact that several parameters have to be fitted together to yield a product that is satisfactory. The parameters have to be found such that, firstly, the insecticidal dose on the surface of the material should be sufficiently high for killing insects, secondly, the release should last for more than a year in the case of LLIN, and should allow regeneration of the net's activity in case of washing of the net, and thirdly, the overall content of the insecticide and synergist should be low in order to keep the cost for LLIN at a minimum, as well as to minimize the potential exposure of an end-user to the active ingredients contained in the net. The latter are important factors, because the largest markets for LLIN are regions with low income, and the primary end-users are infants and young children.
Especially, for insecticidal polymer matrices containing DM and PBO, the optimal ratio between the content of DM relative to the content of PBO in terms of weight is still an open questions.
International patent application WO 2006/111553 discloses microcapsules with a solution of an insecticide, for example DM, and a synergist, for example PBO, in an aqueous suspension inside a polyurea capsule. The ingredients of the microcapsule are released due to breakage of the microcapsule wall when it dries after spraying. The background for this disclosure is the assumption that release of the insecticide should be several hours delayed as compared to the release of the synergist. In the disclosed Example 5, the amount of synergist PBO relatively to the insecticide DM is a factor of 2 in the premix A. As shown in FIG. 1, this premix is then surrounded by an emulsion containing a premix B containing 81% PBO. When used in connection with FIG. 2 of WO 2006/111553, the blend of Premix A and premix B with 5.9% DM and 23.9% PBO yields a ratio of 4.1 between PBO and DM which is further surrounded by premix B containing PBO. Although stating precise figures for containment of PBO and DM in an aqueous suspension inside a microcapsule for delayed release of DM relative to PBO, this disclosure does not give any hint as to the content of PBO relative to DM in a polymer matrix, for example a polyethylene matrix, where the insect shall be exposed to PBO and DM simultaneously by DM and PBO migrated to the surface of the matrix.